If you need to change the email address you use to login to Polymail, you can do so through Polymail Web and macOS apps! (This is not available in the iOS app just yet.)
Your Polymail Login ID should be an email address that exists and you have access to. You will use this email address to login to Polymail on your devices, as well as reset your password. You'll also need it to close your Polymail account if you ever want to. To change your ID:
Click on the account avatar in the lower left corner, then click on Settings in the menu that appears.
Click on the Security tab in the left side of the app. You'll see your security settings as well the email address that you use to log into Polymail.
Click the small arrow in drop down box to change it to another email address you've synced to your account:
You'll be asked to confirm your password to save this change. Enter your password and then click the blue Save button that appears.